1. Field of the Invention
The present invention relates to an apparatus for driving a photosensitive medium in copying machines, printers, recorders or other machines including a photosensitive medium to be driven into rotation (including those photosensitive media in the form of a drum or in the form of a belt). More particularly, the present invention relates to a photosensitive medium driving apparatus which can shift the position of the circumference of the photosensitive medium in the axial direction according to the number of the working rotations of the photosensitive medium.
2. Description of the Prior Art
In the above-mentioned types of machines employing a photosensitive medium, for instance, in an electrophotographic recording machine, the surface of the photosensitive medium is contacted by a cleaning blade under pressure at the cleaning station after transferring the developed image. While maintaining the pressure contact between the drum surface and the cleaning blade, the toner remaining on the drum surface is removed by the cleaning blade. In general, the positional relation between the cleaning blade and the photosensitive drum at the cleaning station is fixed with regard to the axial direction of the drum. This means that the cleaning blade always rubs the drum surface at the same area of the drum surface. Therefore, if the cleaning blade has any torn portion thereon or if there is any foreign matter between the blade and the drum, a scratch may be formed on the drum surface. When such a scratch is formed, it gradually expands over the drum surface as the number of cleanings conducted on the drum surface increases. Consequently, the photosensitive drum is rapidly degraded and its useful life is shortened.
In the case of laser beam printers there is another problem relating to the photosensitive medium.
The problem is that after a number of printing cycles are continuously carried out with the same format, the pattern of the format remains in the photosensitive layer as a memory. After changing the format from one to another, the memorized pattern of the previously used format appears on the print of the new format as a thin pattern image. The memory does not disappear at once but remains for a relatively long time after starting the printing with the new format.
As a solution to the above problems it has already been proposed to shift the photosensitive drum in the axial direction each time the number of prints made reaches a predetermined value. However, this known solution has the following drawbacks:
(1) Since the photosensitive drum is moved stepwise each determined number of prints, it is impossible to erase the memory in it by this solution.
(2) The positional relation between the photosensitive drum and the cleaning blade remains unchanged during the period between one step shift and the next step shift of the drum. Therefore, with regard to this period, the same surface area of the drum is subject to the abrasive force of the blade, which results in the expansion of the scratch previously formed on the surface.
(3) To shift the drum in the axial direction there is required an additional motor and clutch and the control circuits for them. The manufacturing cost of the apparatus rises accordingly. Furthermore, an additional relatively large space is required.
(4) During the time of the drum shift, the printing operation has to be stopped. Therefore, it is impossible to carry out successive printings without interruption.